Underdrilling rotary bit



Dec. 1, 1964 J. M. CLEARY UNDERDRILLING ROTARY BIT Filed Dec. 50'. 1960 4 Sheets-Sheet 1 Fig. 1

INVENTOR.

James M. Cleory BY ATTEST Dec. 1, 1964 J. M. CLEARY UNDERDRILLING ROTARY BIT 4 Sheets-Sheet 2 Filed Dec. 50, 1960 Fig. 3

INVENTOR.

James M. Cleory ATTEST i W Attorney J. M. CLEARY 3,159,224

UNDERDRILLING ROTARY BIT Dec. 1, 1964 Filed Dec. 50, 1960 4 Sheets-Sheet 4 33 E 2? l| l7 37 2 E I I9 9 Fig. 6

39 Fig. 7

INVENTOR. James M. Cleory United States Patent ()filice 3,l%,224 Patented Dec. 1, is

3,159,224 UNBERDRTLLHQG RQTARY BET flames M. Clear" Dallas, Ten, asslgnor to The Atlantic Refining Company, Philadelphia, Pa, a corporation oi Pennsylvania Dec: 35 196i}, Ser. No. 79,687 8 Qlaims. (Cl. l75-391) The present invention relates to an improved bit for drilling boreholes in the earth. In a more particular aspect, the present invention relates to an improved rotary bit adapted to drill a borehole substantially larger than the diameter of the bit itself.

In the drilling of boreholes in the earth, particularly for the production of oil and gas, it is general practice to rotate a drill bit by means of a tubular drill string while, circulating through the interior of the drill string and back to the surface about the exterior of the drill string an appropriate drilling fluid to remove cuttings from the bottom of the borehole. in instances where the drilling fluid is a liquid, there are a number of problems involved in the drilling which must be partially compensated for by the addition of certain agents to the fluid. First, additives must be included in the drilling fluid which will seal the walls of the borehole and prevent loss of drilling fluid into the formations surrounding the borehole and prevent caving of incompetent formations. In addition, weighting materials must be added to the drillig fluid and the pressure on the iiuid maintained sufficiently high to counterbalance the pressure of fluids in the surrounding formations. Rotary drilled holes usually taper slightly during a single bit run due to gage wear on the bit. Sometimes this taper presents the problem of reaming a new bit to bottom. This necessity to ream results in an extra expenditure of rig time and extra bit wear. Alternatively, it may be necessary to make each new bit smaller than the previous bit. On occasion a buildup of cuttings or a combination of mud cake and cuttings on the sidesof the hole wedge and stick the bit as it is pulled from the hole. This difficulty is most common in air or gas drilling when a small amount of moisture is present in the hole.

Following the drilling of the borehole, it is conventional to place in the borehole a tubular casing substantially equal in size to the size of the borehole. This casing is cemented in place and is then used as a conduit for the production of oil or gas. Such production may be from the bottom of the borehole or through perforations in the side of the casing. Due to the fact that bits conventional- 13/ used in present-day operations drill a borehole substantially equal to the diameter of the bit, casing cannot be placed in the borehole until the entire hole has been drilled or the casing must be pulled each time a bit is replaced since the bit is too large to remove or re-insert through casin sized to lit the hole drilled.

Because of the above and other problems encountered in drilling boreholes in the earth, it would be highly advantageous to be able to drill a borehole substantially larger than the diameter of the bit itself. Such a bit would not become wedged or stuck in the hole due to hole taper or buildup of cuttings and mud cake on the walls of the borehole. in addition, it would be possible to follow such a bit with casing and remove and replace the bit through the casing while leav ng the casing in place. Such casing of the hole would have many advantages in controlling the upper portions of the hole already drilled and preventing the entry of water in gas drilling operations.

It is, therefore, an object of the present invention to provide an improved drill bit which will drill a borehole in the earth substantially larger than the diameter of the bit itself.

Another object of the present invention is to provide an improved rotary bit which will drill a borehole in the earth substantially larger than the diameter of the bit itself.

A further object of the present invention is to provide an improved drill bit which can be employed to drill a borehole in the earth while simultaneously running casing in the borehole above the bit.

Another and further object of the present invention is to provide an improved drill bit adapted to enlarge a portion of an existing borehole in the earth.

A still further object of the present invention is to provide an improved drill bit adapted to drill a borehole in the earth substantially larger than the diameter of the bit itself and which can be employed to drill the entire length of the borehole rather than predetermined sections of the hole.

Briefly, in accordance with the present invention, it has been found that a borehole substantially larger than the diameter of a rotary bit can be drilled by providing a bit having a main gage cutting portion adapted to cut the inner perimeter or gage of the hole and a pilot cutting portion offset from the central axis of the main bit body and adapted to drill a pilot hole ahead of the gage cutting portion. The pilot cutting portion of the subject bit is provided with a non-cutting surface on its outer periphcry or side adapted to cause the bit to drift inwardly and a relief cutter adapted to eventually cut relief for the non cutting surface and help stabilize the pilot portion in the center of the hole.

The above objects and advantages of the present invention will be apparent from the following detailed description when read in conjunction with the drawings.

in the drawings:

FIGURE 1 is an elevational view of a comparatively simple rotary drag bit built in accordance with the present invention.

FIGURE 2 is a plan view of the rotary drag bit of FIGURE 1.

FIGURE 3 is an elevational view of a modified rotary drag bit built in accordance with the present invention.

FlGURE 4 is a plan view of the rotary drag bit of FIGURE 3.

FIGURE 5 is a series of views showing the manner in which the bit of the present invention functions to drill a borehole substantially larger than the diameter of the bit.

FIGURES 6 and 7 are fragmented elevational views of the rotary drag bit of FIGURE 1 showing different locations of the pilot portion of the bit and different slopes for the gage cutter.

As used herein, the term axis or axis of revolution of any member of the bit is located at the point about which that member could be rotated so that its outermost extremities would circumscribe the smallest possible circle. The term periphery or outer side edge of any member of the bit means all outer sides or edges of the member of the bit coincident with or just lying in a curved line bounding the surface or edge which curved line circumscribes the smallest circle than can be formed by rotating that element or member of the bit. The term diameter or over-all width of any member of the bit is the length of the diameter of the smallest possible circle circumscribing the outermost extremities of the member of the bit.

Referring specifically to FIGURES 1 and 2 of the drawings, these drawings show a rotary drag bit built in accordance with the present invention and designed to rotate in a clockwise direction when viewed from the top of the borehole. The bit of FIGURES 1 and 2 includes a main bit body 1 which is threaded at its upper end 3 for attachment to a conventional tubular drill string (not shown). Main bit body 1 has a central axis 5 about which the bit would normally rotate. Mounted on bit body 1 are vertically disposed cutters 7, 9 and ll. Cutters 7, 9 and 11 may be divided into two portions, as shown by the horizontal dashed line in FIGURE 1, to form an upper gage cutting portion 13 and a lower pilot cutting portion 15 As FIGURE 1 further illustrates, pilot cutting portion. 15 leads gage cutting portion 13 by a predetermined distance in the direction of bit penetration. Since the main cutting force of the subject drag bit is a rotary motion, cutting blades 7 and 9 are provided with cutting edges 33 on their leading face which then taper inwardly and rearwardly with relation to the direction of rotation of the bit to form conventional cutting faces 17. In contrast to these cutting faces 17, blade 11 is provided with noncutting surface 19 which is formed oppositely from cutting faces 17 (sloping inwardly and forwardly with relation to the direction of rotation). Noncutting surface 19, being in an opposite direction to cutting faces 17, presents a noncutting surface as the bit is rotated in its normal direction and, therefore, does not cut into the walls of the borehole as it would conventionally do. The pilot cutting portion 15 of blades '7 and 9 also has formed thereon cutting faces 21 and cutting edges 35. Thus, relief cutting portions on pilot cutting portion 15 formed by cutting edges 3-5 of blades 7 and 9 will cut into the side walls of the pilot hole in a manner similar to cutting faces 17. It is to be noted that cutting edges 35 of cutting faces 21 extend outwardly a sufficient distance so that they will cut a cylinderwhose circumference includes noncutting surface 19 of blade 11, as shown by small dashedcircle w of FIGURE 2. It shouldalso be noted that the central axis 23 of pilot portion 15 is offset from axles. of the main bit body. The bottom faces of blades 7, 9 and 11 are all inclined upwardly and rearwardly with rclationto the direction of rotation of the bit so as to provide sharp cutting edges 37 and 39 on their leading face which willcut into the bottom of the hole as weight is applied to the bit and the bit is rotated. Although the body 1.

bottom edges of cutters 7, 9 and 11 are shown to lie in horizontal planes, it should be recognized that these cutting edges may be inclined upwardly and outwardly or upwardly and inwardly (as shown in FIGURES 1, 3, 6 and 7). Located in the bottom of bit body 1 are conventional drilling fluid ports 25 through which drilling fluid from the drill string may wash across the bottom of the bit and remove cuttings as the drilling progresses. Blades '7, 9 and 11 of the bit may be welded to body 1 or attached in any other suitable manner. From the standpoint of strength, the three blades may be welded together at their centers as shown by weld 27.

As will be explained in more detail hereinafter, the bit of FIGURES l and 2 will drill a hole substantially larger than the bit and ultimately become stabilized with the axis' 23 of pilot portion 15 coincident with the central axis of the borehole and gage portion 13 revolving about this axis. The ultimate gage of the borehole being drilled when the bit has reached its stabilized position is illustrated by large dashed circle x of FIGURE 2.

FIGURES 3 and 4 of the drawings illustrate a modification of the rotary drag bit of FIGURES 1 and 2. InFIGURES-3 and 4, the same numbers are employed to designate corresponding items in order to emphasize that and 9, cutting faces 17 with cutting edges 33 are present on each step. Similarly, cutting'faces 2.1 with cutting edges 35 are formed on each step of blades 7 and 9 which form part of pilot portion 15 of the bit. Blade 11 has noncutting'surface 19 formed on its'steps. As in the bit surface.

. 4. of FIGURES 1 and 2, the bottoms of all blades are sloped upwardly and rearwardly with relation to the direction of rotation of the bit to form cutting edges 3'7 and 39. One additional distinction between the bit of FIGURES 3 and 4 and that or" FIGURES 1 and 2 is the fact that the lowermost edge of gage cutting portion 15 of blades 7, 9 and 12. are formed in the shape of a l as illustrated by points 29 of blades 7, 9 and 11, respectively. By comparing the modified bit with the previously described bit, it will be obvious that the former will operate in substantially the same way as the bit first described and will drill a borehole substantially larger than the bit itself.

Although the bits illustrated in FIGURES 1 through 4 show the outer periphery or side edge of pilot portion 15 most remote from the axis of revolution of the bit body extending beyond the outer periphery or side of the bit body and coincident with the outer edge of the over-all bit, this placement of, pilot portion 15 is not necessary to the functioning of the bit in its intended manner. It should, therefore, be recognized that pilot portion 15 can be moved in toward the center of the bit, as shown in FEG RE 6 wherein that part of the outer periphery or side edge of the pilot portion most remote from the axis of revolution of the bit body is coincident with the outer periphery or side of bit body 1 and in FIGURE 7 wherein that part of the outer periphery or side edge of the pilot portion most remote from the axis of revolution of the bit body is inside the outer periphery or side of bit if this is done, the bit will drill a smaller hole since the maximum diameter of the hole drilled is equal to twice the distance from central axis 23 of pilot por-,

tion 15 to the tips of the blades of gage cutting portion 13; V or, stated differently, the maximum amount by which the iarncter of the hole exceeds the diameter or" the main bit is twice the amount by which central axis 23 is offset from central axis 5 of the main bit. Also, if pilot portion 15 is moved toward the center of the bit, the portion of gage cutting portion 13 which protrudes beyond pilot portion 15 may be provided with a cutting or a noncutting in this case, it is obvious that the peripheral cutting edge will serve no useful purpose after the bit becomes stabilized. This protruding portion of gage cutting portion 13 may or may not have a cutting edge on its base, although such a cutting edge can be designed to help the cutting. action of the other gage cutters.

The distance by which pilot portion 15 leads gage cutting portion 13 is important tothe operation of the bit. If pilot portion 15 is too short, the pilot hole will not be deep enough to stabilize pilot portion 15 and pilot portion 15 will continuously destroy the shoulders of the pilot hole and the bit will drill a smaller hole fluctuating in size and in some cases out of round. Therefore, the ratio of the length of pilot portion 15 to the diameter or width of the over-all bit should be at least about 0.15. In addition, if pilot portion i5 is too long, the pilot blades will tend to bend or break readily. Accordingly, the ratio of the length of. pilot portion 15 to the diameter or width of the over-all bit should be less than about 1.

Although the drawings of FIGURES 1 through 4 do not bring out the point, the efiectiveness of the lateral force means, or noncutting surface 19, in forcing pilot portion 15 toward the center of the hole and stabilizing the bit in this position will be substantially improved by also sloping noncutting surface 19 inwardly and forwardly, as shown in FIGURE 5, with relation to the direction of penetration of the bit. This angle of slope should, however, be comparatively small.

As will be explained hereinafter, a single blade may be utilized on gage cutting portion 13 rather than a plural-' ity of blades as shown in FIGURES 1 through 4. However, a single blade is seriously limited in use in that a ters shouldl be employed in accordance with the present invention. If two or more blades are mounted on gage cutting portion 13, certain critical design features enter the picture. Since the basic objective of the present invention is to provide a bit which will pass through a comparatively small hole, such as a casing, and drill a substantially larger hole, the main factor in the design of the present bit is the amount or percent of hole enlargement obtained over the size of the bit itself. The greatest percent of hole enlargement over the bit size will be obtained when a single gage cutter is used and the peripheral cutting edge of this cutter and the outermost peripheral point of noncutting surface 19 are separated by 180. However, as previously pointed out, a bit with a single gage cutter is seriously limited. It has been found that the spread or angle between the tips of the outermost gage cutters of the bit should be below a certain critical limit when two or more gage cutters are employed. This lower limit as applied to all bits, irrespective of the size of the pilot portion, the location of the central axis of the pilot or the location of the point from which the cutters radiate, is best expressed by a comparison of the distance y (FIGURE 2) between the tips of the two outermost gage cutters and the diameter of the bit; that is, the diameter of the smallest circle 2 (FIGURE 2) which can be drawn enclosing the entire bit. In accordance with the present invention, the ratio of the distance between the points of the two outermost gage cutters to the diameter of the bit should be less than 0.95. V-Ihere the bit is to be employed to underdrill below a casing, this ratio should preferably be below 0.80. By utilizing this criteria in the design of the bit, the percent hole enlargement will be substantial; whereas, for a larger ratio (a larger ratio of 3 /2 than 0.95), the percent hole enlargement will be almost negligible. Depending upon the distance between the points of the two outermost gage cutters, it may be desirable in some cases to add additional gage cutters. This, however, depends upon whether the cutters can be spaced a sufficient distance apart to permit the removal of cuttings and prevent clogging of the bit.

In order to give a specific example or" the application of the above criteria to the design of a bit, reference is made to FIGURE 2 of the drawings. In FIGURE 2, the difl'erence between the diameter of the bit, as represented by dashed circle 2, and the diameter of the borehole it will drill, as depicted by dashed circle x, divided by the diameter of the bit gives a hole enlar ement of approximately 45 percent. This design will thus serve to underdrill below a casing since the enlargement should be above about 30 percent for this purpose. The ratio of the dis tance y between the tips of blades 7 and 9 over the diameter of the bit, as represented by circle 2, is about 0.81. Since blades 7 and radiate from the central axis of pilot portion of the bit, blades '7 and 9 form an angle of approximately 65. However, this angle would change if the point from which blades 7 and 9 radiate were moved inwardly or outwardly. Accordingly, the ratio of the distance between the points of the cutter to the diameter of the overall bit is a better criteria than the angle between the two gage cutters. However, in general, all bits designed within the minimum set forth above (ratio of 0.95) will have an angle between the blades of less than about 120 and when designed Within the preferred limit (ratio of 0.80) this angle will be less than about 90.

A related factor, which also bears on the percent of hole enlargement which can be attained, is the comparative diameter of the over-all bit (diameter of circle z) and the diameter of the pilot (diameter of circle w). Since gage cutting portion 13 of the bit sweeps about axis 23 of pilot 15 when the bit becomes stabilized the ultimate gage of the hole will depend upon the ratio of the bit diameter to the pilot diameter. If this ratio is 1.0, the bit obviously will not drill an enlarged hole; and, the larger this ratio the greater will be the percent hole enlargement. However, a pilot which is too small as compared with the over-all bit will be too weak to function properly or will tend to wear unevenly or break frequently. Accordingly, it has been found that, as a practical matter, the ratio of the diameter or width of the over-all bit to the width or diameter of the pilot should not exceed 3. Further, in order to underdrill below a casing, this ratio should be above about 1.3.

Although it has previously been indicated that the outer periphery or outer side edge of the pilot need not be coincident with the outer periphery of the main bit, good design dictates that such coincidence be present.

Finally, in order to achieve maximum cutting elliciency, the pilot cutters should preferably radiate from the axis of the pilot since this is the axis about which the entire bit rotates when it reaches its stabilized position.

In order to illustrate the manner in which the bit of the present invention drills a hole, substantially larger than the bit itself, reference is made to FIGURE 5 of the drawings. In order to simplify the following discussion and the drawings of FIGURE 5, it is assumed that blade 11 extends across the entire pilot portion of the bit. A single blade of this character will function in accordance with the present invention so long as the inner tip of the blade has a cutting face 21 formed thereon and the inner extension of the blade thus cuts a cylinder whose circumference includes noncutting surface lit. Similarly, gage cutting portion 13 of blades 7 and 9 can be replaced by a single blade across the middle of gage cutting portion 13 so long as the outermost tip of this blade is designed with a cutting face 37.

View a of FlGURE 5 shows the bit disposed in a borehole substantially the same size as the bit itself and before drilling is commenced. It is to be noted that in view a central axis 5 of the main bit body conicides with central axis 31 of the bore hole. In contrast, central axis 23 of pilot cutting portion 15 of the bit is oifset from axis 31 of the borehole. View b shows the position of the bit after the bit has drilled some distance downwardly. In this view, noncutting surface 19 of pilot portion 15 has forced pilot portion 15 toward the center of the borehole as the bit was rotated. This comes about because of the fact that as the bit is rotated noncutting surface 19 will not cut into the side of the formation and will thus apply a lateral force to pilot cutting portion 15 forcing it to pinch in toward the center of the hole. Accordingly, axis 23 of pilot portion 15 has drifted in toward axis 31 of the borehole and axis 5 of the main bit body has drifted away from axis 31 of the borehole and is now sweeping about axis 31. As a result of this displacement of the bit, gage cutting portion 13 has begun to enlarge the hole by an amount equal to twice the shift of the axes. View 0 shows the position of the bit after further penetration. In view c, axis 23 of pilot portion 15 has shifted in closer to borehole axis 31, axis 5 of the main bit body has shifted further away from borehole axis 31 and gage cutting portion 13 has further enlarged the major dimension of the hole. Just as the continuous inward drift of pilot portion 15' has caused gage cutting portion 13 to continuously enlarge the major dimension of the hole, pilot portion began to drill a hole much larger than pilot portion 15, as shown in view 5;, which pilot hole has continuously decreased, as shown in view 0. Finally, view at shows the bit in its final position where it will become stabilized} It should be noted that when the bit is in its stabilized position, axis 23 of pilot portion 15 coincides with axis 31 of the borehole and axis 5 of the main bit body is still further displaced from borehole axis 31. The bit has become stabilized in this position due to the fact that the cutting edge on cutting face 21 of gage cutting portion 15 is now cutting a cylinder whose circumference includes noncutting surface 19 or, in other words, cutting faces 21 of blades 7 and 9 are cutting relief for noncutting surface 19. Also in this position, the entire bit is rotating about axis 23 of pilot portion 15 and gage cutting portion 13 is cutting a hole whose maximum diameter is twice the distance from axis 23 of pilot portion 15 to the outermost tip of blades 7 and 9 of gage cutting portion 13.

Although specific illustrations of rotary drag bits have been given herein, it is to be recognized that various modifications will be apparent to those skilled in the art which can be utilized in the building of other types of rotary bits incorporating the basic features of the present invention. Accordingly, the present invention is to be limited only in accordance with the appended claims.

I claim:

1. An improved rotary drag bit for drilling boreholes in the earth comprising a main bit body, rotary pilot drilling means extending from the bottom of said bit body whose cross section is smaller than said bit body and whose vertical axis of revolution is oflset from the vertical axis of revolution of said bit body, a sloping noncutting surface formed on the side of said rotary pilot drilling means furthest from said axis of said bit body and adapted to force said rotary pilot drilling means inwardly toward said axis of said bit body, at

least one first rotary drag-type cutter on said rotary pilot drilling means whose bottom end terminates in a cutting edge formed by a surface sloping upwardly and rearwardly with relation to the direction of rotation of said bit and whose side edge nearest said axis of revolution of said bit body terminates in a cutting edge formed by a surface sloping inwardly and rearwardly with relation to said direction of rotation of said bit, said first rotary drag-type cutter being adapted to drill a cylindrical .pilot hole Whose circumference includes the outermost extremity of said sloping noncutting surface when said axis of said rotary pilot drilling means coincides with the central axis of said borehole, and at least one second rotary drag-type cutter formed on said bit body above said first rotary drag-type cutter on said n C1 tion to said direction of rotation of said bit, said at least one second rotary drag-type cutter being adapted to cut an annular section of the earth immediately surrounding the hole drilled by said rotary pilot drilling means, and the ratio of the length of the rotary pilot drilling means to the over-all width of said bit being sufiicient to stabilize the position of said bit in said borehole when said axis of said pilot drilling means coincides with the central axis of said borehole.

2. A bit in accordance with claim 1 wherein the outer side edge of the rotary pilot drilling means most remote from the axis of revolution of the bit body is coincident with the outer edge of the over-all bit.

3. A bit in accordance with claim 1 wherein the outer side edge of the rotary' pilot drilling means most remote from the axis of revolution of the bit body is located inside the edge of the over-all bit. I

4. A bit in accordance with claim 1 wherein the sloping vnoncutting surface slopes inwardly and forwardly with relation to the direction of rotation of the bit.

5. A bit in accordance with claim 1 wherein the cutting edge on the bottom face of the first rotary drag-type cutter is V-shaped with the point of the V near the axis of revolution of the pilot means.

6. A bit in accordance with claim 1 wherein the cutting edge on the bottom face of the second rotary dragtype cutter slopes upwardly and outwardly toward the I periphery of the bit body.

7. A bit in accordance with claim 1 wherein the over all width of the bit is between 1.3 and 3.0 times as great as the width of the rotary pilot drilling means.

8. A bit in accordance with claim 1 wherein the ratio of the length of the rotarypilot drilling means to the over-all width of bit is between 0.15 and 1.0.

References tilted in the file of this patent UNITED STATES PATENTS Great Britain Sept. 15, 1942 

1. AN IMPROVED ROTARY DRAG BIT FOR DRILLING BOREHOLES IN THE EARTH COMPRISING A MAIN BIT BODY, ROTARY PILOT DRILLING MEANS EXTENDING FROM THE BOTTOM OF SAID BIT BODY WHOSE CROSS SECTION IS SMALLER THAN SAID BIT BODY AND WHOSE VERTICAL AXIS OF REVOLUTION IS OFFSET FROM THE VERTICAL AXIS OF REVOLUTION OF SAID BIT BODY, A SLOPING NONCUTTING SURFACE FORMED ON THE SIDE OF SAID ROTARY PILOT DRILLING MEANS FURTHEST FROM SAID AXIS OF SAID BIT BODY AND ADAPTED TO FORCE SAID ROTARY PILOT DRILLING MEANS INWARDLY TOWARD SAID AXIS OF SAID BIT BODY, AT LEAST ONE FIRST ROTARY DRAG-TYPE CUTTER ON SAID ROTARY PILOT DRILLING MEANS WHOSE BOTTOM END TERMINATES IN A CUTTING EDGE FORMED BY A SURFACE SLOPING UPWARDLY AND REARWARDLY WITH RELATION TO THE DIRECTION OF ROTATION OF SAID BIT AND WHOSE SIDE EDGE NEAREST SAID AXIS OF REVOLUTION OF SAID BIT BODY TERMINATES IN A CUTTING EDGE FORMED BY A SURFACE SLOPING INWARDLY AND REARWARDLY WITH RELATION TO SAID DIRECTION OF ROTATION OF SAID BIT, SAID FIRST ROTARY DRAG-TYPE CUTTER BEING ADAPTED TO DRILL A CYLINDRICAL PILOT HOLE WHOSE CIRCUMFERENCE INCLUDES THE OUTERMOST EXTREMITY OF SAID SLOPING NONCUTTING SURFACE WHEN SAID AXIS OF SAID ROTARY PILOT DRILLING MEANS COINCIDES WITH THE CENTRAL AXIS OF SAID BOREHOLE, AND AT LEAST ONE SECOND ROTARY DRAG-TYPE CUTTER FORMED ON SAID BIT BODY ABOVE SAID FIRST ROTARY DRAG-TYPE CUTTER ON SAID ROTARY PILOT DRILLING MEANS WITH RESPECT TO THE DIRECTION OF PENETRATION OF SAID BIT, SAID AT LEAST ONE SECOND ROTARY DRAG-TYPE CUTTER HAVING A BOTTOM END TERMINATING IN A CUTTING EDGE FORMED BY A SURFACE SLOPING UPWARDLY AND REARWARDLY WITH RELATION TO SAID DIRECTION OF ROTATION OF SAID BIT AND SIDE EDGE MOST REMOTE FROM SAID AXIS OF SAID BIT BODY TERMINATING IN CUTTING EDGE FORMED BY A SURFACE SLOPING INWARDLY AND REARWARDLY WITH RELATION TO SAID DIRECTION OF ROTATION OF SAID BIT, SAID AT LEAST ONE SECOND ROTARY DRAG-TYPE CUTTER BEING ADAPTED TO CUT AN ANNULAR SECTION OF THE EARTH IMMEDIATELY SURROUNDING THE HOLE DRILLED BY SAID ROTARY PILOT DRILLING MEANS, AND THE RATIO OF THE LENGTH OF THE ROTARY PILOT DRILLING MEANS TO THE OVER-ALL WIDTH OF SAID BIT BEING SUFFICIENT TO STABILIZE THE POSITION OF SAID BIT IN SAID BOREHOLE WHEN SAID AXIS OF SAID PILOT DRILLING MEANS COINCIDES WITH THE CENTRAL AXIS OF SAID BOREHOLE. 